Metal wires conveyed along a longitudinal axis serve, for example, as conductors of electrical cables. They possess a diameter within the range of 0.1 to 100 mm and are conveyed by appropriate guide apparatus along their longitudinal direction, for example to extrusion devices in which an insulation sheath is applied to the metal wires. Inter alia for the extrusion process but also, e.g., for a preceding preheating, curing or hardening process, it is necessary for the moved metal wires to possess a set temperature. In the production of cores, it is important for the insulation to adhere to the connector. To this end, the conductor is heated directly before entering the extruder. In addition, a constant conductor temperature is particularly important when the insulation of a conductor is foamed. This is particularly the case with data transmission lines.